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Motor Schema - Based Mobile Robot Navigation System

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The concept of schema has its origin in psychology, neuroscience and brain ... of the robot is thought of as a conglomeration of several behaviors i.e. schemas. ... – PowerPoint PPT presentation

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Title: Motor Schema - Based Mobile Robot Navigation System


1
Motor Schema - Based Mobile Robot Navigation
System
  • - Ronald C. Arkin.

2
What is a schema ??
The concept of schema has its origin in
psychology, neuroscience and brain theory. It is
based on the action - perception cycle. There are
several possible definitions for schemas
To put it more concisely, a schema is a behavior.
3
Problem addressed by the paper
  • The environment surrounding the robot is subject
    to changes
  • Traditional control structures those that use
    an inflexible and rigid approach to navigation
    do not provide the essential adaptability
    necessary for coping with unexpected events.
    These events may include unanticipated obstacles
    or moving objects. These unexpected occurrences
    should influence, in an appropriate manner, the
    course that a vehicle takes in moving from start
    to goal.

4
Background
  • Schemas is a methodology used to describe the
    interaction between perception and action.

Sensor driven expectations
Plans (schemas)
Motor action
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  • This cycle of cognition (the altering of
    internal world model), direction (selection of
    appropriate motor behaviors), and action (the
    production of environmental changes and resultant
    availability of new sensory data) is central to
    the way schemas interact with the world.
  • Significantly, perception should be viewed as
    action oriented and there is no need to process
    all the available sensor data, only the data that
    is pertinent to the task at hand.
  • The question as to which sensor data should be
    considered relevant can be answered as follows
    The overall behavior of the robot is thought of
    as a conglomeration of several behaviors i.e.
    schemas.
  • By specifying the schemas, each individual
    component of the overall task can make its
    demands known to the sensory subsystem and thus
    limit the sensory information processed.

6
Schema Instantiation (SI)
  • Each schema represents a generic behavior. The
    instantiations of these generic schemas provide
    the potential actions for the control of the
    robot. A schema instantiation (SI) is created
    when a copy of a generic schema is parameterized
    and activated as a computing agent.

7
Key ideas
  • Potential fields are used to produce steering
    commands for a mobile robot.
  • The idea of using individual SIs for each
    obstacle and associating an uncertainty with each
    of them is used.
  • Additional behaviors such as road following and
    treatment of moving obstacles have been
    incorporated.
  • The state of each obstacles SI is dynamically
    altered by newly acquired sensory information.
  • The potential functions for each SI reflect the
    measured uncertainty associated with the
    perception of each object.

8
Approach
  • Each motor schema has an embedded perceptual
    schema (an identification procedure) to provide
    the necessary sensor information for the robot to
    relate to its world. Motor schemas are
    instantiated based on the outputs from the
    perceptual schemas.
  • Motor schemas when instantiated must drive the
    robot to interact with its environment.
  • On the highest level this will be to satisfy a
    goal developed within the planning system on the
    lowest level, to produce specific translations
    and rotations of the robot vehicle.

9
Schema based Navigation
  • AuRAs pilot is charged with implementing
    leg-by-leg the piecewise linear path developed by
    the navigator.
  • The pilot chooses from a range of available
    sensing strategies and motor behaviors (schemas)
    and passes them down to the motor schema manager
    for instantiation.
  • Distributed control and low level planning occur
    within the confines of motor schema manager
    during its attempt to satisfy navigational
    requirements.
  • As the robot proceeds, AuRAs cartograher, using
    sensor data, builds up a model of the perceived
    world in short term memory.

10
Schema based Navigation
  • If the actual path deviates too much from the
    path initially specified by the navigator as a
    result of presence of unmodeled obstacles or
    positional errors, the navigator will be
    reinvoked and a new global path computed.
  • But if the deviations are within acceptable
    limits, the pilot and motor schema manager will,
    in a coordinated effort, attempt to bypass the
    obstacle, follow the path, or cope with other
    problems as they arise.

11
Schema based Navigation
  • Multiple concurrent behaviors can be present
    during any leg, for example
  • Stay on path
  • Avoid static obstacles
  • Avoid moving obstacles
  • Find intersection
  • Find landmark
  • The first three are examples of Motor schemas and
    the last two are examples of perceptual schemas.
  • To provide the correct behavior, a subset of
    perceptual schemas must be associated with each
    motor schema.

12
Schema based Navigation
  • An example illustrating the relationship between
    motor schemas and perceptual certainty follows.
  • A robot is moving across a field in a particular
    direction (move ahead schema). The find obstacle
    schema is constantly on the lookout for possible
    obstacles within the subwindow of the video
    image.
  • When an event occurs (e.g. an area distinct from
    the surrounding backdrop is detected), the find
    obstacle schema spawns off an associated
    perceptual schema (static obstacle SI) for that
    portion of the image.

13
Schema based Navigation
  • It is now the static obstacle SIs responsibility
    to continuously monitor that region.
  • Any other events that occur elsewhere in the
    image spawn off separate static obstacle Sis.
  • Additionally an avoidstaticobstacle SI motor
    schema is created for each detected potential
    obstacle.
  • The motor schema SI hibernates, waiting for
    notification that the perceptual schema is
    sufficiently confident in the obstacles
    existence to warrant motor action.

14
Schema based Navigation
  • If the perceptual schema proves to be a phantom
    (e.g. a shadow) and not an obstacle at all, both
    the perceptual and related motor SIs are
    deinstantiated before producing any motor action.
  • On the other hand if the perceptual SIs
    confidence exceeds the motor SIs threshold for
    action, the motor schema starts producing a
    repulsive field surrounding the obstacle.
  • The sphere of influence (spatial extent of
    repulsive forces) and the intensity of repulsion
    of the obstacle are affected by the distance from
    the robot and the obstacles perceptual certainty.

15
Schema based Navigation
  • Eventually, when the robot moves beyond the
    perceptual range of the obstacle, both the motor
    and perceptual SIs are deinstantiated.
  • In summary, when obstacles are detected with
    sufficient certainty, the motor schema associated
    with a particular obstacle (its SI) starts to
    produce a force tending to move the robot away
    from the object.

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Simulation
  • Simulations were run on a VAX 750 using the
    following motor schemas stay on path, move
    ahead, move to goal, avoid static obstacle.
  • An example simulation run shows the sequence of
    resultant overall force fields based on perceived
    entities. In this example all obstacles are
    modeled as circles.

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Simulation
  • The field equations for several of the motor
    schemas are calculated as follows

27
Simulation
  • In this simulation, the uncertainty in perception
    was allowed to decrease the sphere of influence
    of an obstacle.
  • When a threshold was exceeded (50 certain) the
    sphere of influence of the obstacle started
    increasing linearly as the certainty increased,
    up to its maximum allowable value.
  • When the robot has successfully navigated
    obstacles and they have moved out of range, their
    representation is dropped from short-term memory,
    and the associated motor schema is
    deinstantiated.

28
Experiments in Motor Schema Based Navigation
  • The experiments were performed using the mobile
    robot HARV (a Denning Research Vehicle), based on
    sensing using ultrasonic and encoder data.
  • Five different motor schemas were implemented
    move ahead (encoder based), move to goal (encoder
    based), avoid static obstacle (ultrasonic based),
    follow the leader (ultrasonic based), and noise
    (sensor independent).

29
Experiment 1 (Avoidance)
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Experiment 2 (Navigation in presence of obstacles)
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Experiment 3 (Single Wall Following)
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Experiment 4 (Impatient Waiting)
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Conclusions
  • Motor schemas serve as a means for reactive/
    reflexive navigation of a mobile robot.
  • The schema based methodology has many advantages,
    including the use of distributed processing,
    which facilitates real time performance, and
    the modular construction of schemas for ease in
    the development, testing, and debugging of new
    behavioral and navigational patterns.
  • Complex behavioral patterns can be emulated by
    the concurrent execution of individual primitive
    SIs.

39
Conclusions
  • The use of velocity fields to reflect the
    uncertainty associated with a perceptual process
    is another important advance.
  • By allowing the force produced by a perceived
    environmental object to vary in relationship to
    the certainty of the objects identity (whether
    it be an obstacle, goal path, or whatever),
    dynamic replanning is trivialized.
  • Since the sensed environment produces the forces
    influencing the trajectory of the robot, when the
    perception of the environment changes, so do the
    forces acting on the robot, and consequently so
    does the robots path.
  • All this is accomplished at a level beneath the
    prior knowledge representations.

40
Thank you.
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